The present invention relates to a radially anisotropic ring magnet with improved linearity and/or peak value in a space magnetic flux density distribution than those of conventional ring magnets, and a speaker comprising such a radially anisotropic ring magnet for having improved linearity and/or peak value in thrust of a voice coil.
Speakers of moving coil type have conventionally been used widely. A moving coil-type speaker is a speaker comprising a magnet and a yoke for generating a thrust for moving a voice coil in a magnetic gap, the voice coil coupled with a vibration system being movably disposed in the magnetic gap, and a driving current is caused to flow through the voice coil to generate sound.
FIG. 6 is a cross-sectional view showing an important part of a conventional moving coil-type speaker. In FIG. 6, a frame 1 formed by die-cast aluminum, etc. comprises a substantially conical upper frame 1a and a substantially arm-shaped lower frame 1b coupled with each other by screws 1c. The lower frame 1b is integrally provided with a cylindrical projection 1d at center, and a cylindrical inner yoke 7 made of a ferromagnetic material such as iron is fixed to an outer surface of a small-diameter portion 1e at a tip end of the projection 1d. Two voice coils 6a, 6b wound in opposite directions are closely fixed to an outer surface of the inner yoke 7 with a gap therebetween in a vertical direction. Disposed around the outer surfaces of the voice coils 6a, 6b with a slight magnetic gap are radially magnetized ring magnets 5a, 5b. The ring magnet 5a is magnetized such that its inner surface has an N pole and its outer surface has an S pole. The ring magnet 5b is magnetized such that its inner surface has an S pole and its outer surface has an N pole. The outer surfaces of the ring magnets 5a, 5b are adhered to the inner surface of the cylindrical outer yoke 4.
The ring magnets 5a, 5b used in the speaker shown in FIG. 6 are magnetized radially, and this speaker can avoid damage to its vibration system due to excess vibration generated when excess current flows through the voice coil, without needing a special safety gear. In the moving coil-type speaker, a driving current is enhanced to increase a stroke of the vibration system, to obtain a sound pressure in a low sound region on the same level as those in middle and high sound regions. To increase the stroke of the vibration system, increase in the linearity and/or peak value of the thrust of the voice coil is effective, desirable for satisfying the recent demand for miniaturization and increase in performance of speakers.
However, when a driving current is increased to enlarge the thrust of the voice coil, heat generated from the voice coil increases in proportion to the driving current. Thus, the temperature elevation (burning) of the voice coil should be prevented by limiting electric power supplied to the speaker and improving the heat dissipation of the speaker. Therefore, it is actually difficult to increase the thrust of the voice coil. It has also been found that when a moving coil-type speaker is constituted by conventional ring magnets 5a, 5b, linearity and/or peak value cannot fully be increased in an effective space magnetic flux density distribution crossing the voice coil movably disposed in the magnetic gap.
Accordingly, an object of the present invention is to provide a speaker comprising a radially anisotropic ring magnet for providing improved linearity and/or peak value in the thrust of a voice coil as compared to conventional ones.
As a result of intense research in view of the above objects, the inventors have found that a radially anisotropic ring magnet with improved linearity and/or peak value in a space magnetic flux density distribution is obtained by providing a plurality of radially anisotropic regions along a center axis of the ring magnet, and by making a radial anisotropy direction in each region different from each other, and thus achieving the present invention.
The radially anisotropic ring magnet according to one embodiment of the present invention comprises at least one first radially anisotropic region having a radial anisotropy direction of 89xc2x0 or more relative to a center axis thereof; and at least one second radially anisotropic region having a radial anisotropy direction of 40xc2x0 or more and less than 89xc2x0 relative to a center axis thereof, the first and second radially anisotropic regions being arranged along the center axis such that a space magnetic flux density distribution on an inner or outer surface of the ring magnet has increased linearity and/or peak value.
The radially anisotropic ring magnet according to another embodiment of the present invention comprises a plurality of radially anisotropic regions having radial anisotropy directions of 40xc2x0 or more and less than 89xc2x0 relative to a center axis thereof, the plurality of radially anisotropic regions being arranged along the center axis such that a space magnetic flux density distribution on an inner or outer surface of the ring magnet has increased linearity and/or peak value.
From the practical point of view, the above ring magnet is preferably made of an Rxe2x80x94Txe2x80x94B permanent magnet having as a main phase an R2T14B intermetallic compound, wherein R is at least one rare earth element including Y, at least one of Nd, Dy and Pr being indispensable, and T is Fe or Fe and Co.
The present invention also provides a speaker comprising the above ring magnet.